The present invention relates to a copper-based alloy excellent in shape memory properties and superelasticity, members such as a wire, plate, foil, pipe, etc. made of the copper-based alloy, and methods for producing the copper-based alloy or the members.
It has been well known that shape memory alloys such as Tixe2x80x94Ni alloys, copper-based alloys, etc. exhibit a remarkable shape memory properties and superelasticity by the inverse transformation of martensitic transformation. The Tixe2x80x94Ni alloys are excellent in the shape memory properties and superelasticity at the vicinity of a daily life surrounding temperature to have been widely used in various applications such as dampers for microwave ovens, wind-controllers of air conditioners, steam pressure-controlling valves of rice cookers, air-vents for architectures, antennas of cellular phones, glass frames, brassiere flames, etc. Though the Tixe2x80x94Ni alloys are superior in many aspects such as repeatability and corrosion resistance to the copper-based alloys, the Tixe2x80x94Ni alloys are more than 10 times expensive as the copper-based alloys. Thus, it is desired to develop a less expensive, superelastic, shape memory alloy.
Under these circumstances, such studies as to put the copper-based shape memory alloys with cost advantages into practical use have been carried out. However, most of the conventional copper-based alloys are poor in cold-workability, cannot be cold-worked at a cold-working ratio of or more (Shape Memory Materials, Cambridge press, 1998, p.143), thereby being far from practicable. Therefore, researches for making the crystal grains composing the alloys fine to improve its cold-workability and mechanical properties have been in progress. The inventors previously proposed Cuxe2x80x94Alxe2x80x94Mn shape memory alloys excellent in the cold-workability, having a xcex2-single phase structure (Japanese Patent Laid-Open No. 7-62472).
The above-mentioned Cuxe2x80x94Alxe2x80x94Mn shape memory alloys have excellent shape memory properties and superelasticity. However, with respect to the alloy, a maximum strain providing a shape recovery ratio of 90% or more is approximately 2 to 3% as well as the conventional copper-based alloys, and its superelasticity is often insufficient. The reason for the insufficient superelasticity seems that the Cuxe2x80x94Alxe2x80x94Mn alloys are produced by conventional cold-working at the working-ratio of less than 30% without improving a crystal orientation of the alloy, thereby failing to obtain preferable crystal orientation.
Accordingly, an object of the present invention is to provide a copper-based alloy having high shape memory properties and superelasticity while maintaining an excellent workability, members such as a wire, plate, foil, pipe, etc. made of the copper-based alloy, and methods for producing the copper-based alloy or members.
As a result of intense research in view of the above object, the inventors have found that the shape memory properties and superelasticity of a copper-based alloy can be extremely improved by a method for making crystal grains fine different from conventional methods. Thus, the inventors have found that the shape memory properties and superelasticity of the copper-based alloy is remarkably improved by aligning crystal orientation of xcex2-single phase composing a microstructure of the copper-based alloy, and that a maximum cold-working ratio in cold-working and conditions of a solution treatment affect to the crystal orientation of xcex2-single phase, and further that the larger an average grain diameter in the xcex2-single phase, the better the shape memory properties are. Furthermore, the inventors have found that in the case of producing members such as a wire, plate, foil, pipe, etc. from the copper-based alloy, the shape memory properties and superelasticity of the members can be improved by setting the conditions of the solution treatment such that the average grain diameter in the xcex2-single phase is equal to or more than the radius or thickness of the members. The present invention has been accomplished by these findings.
Thus, a copper-based alloy according to the present invention, which has shape memory properties and superelasticity, has a recrystallization structure substantially composed of xcex2-single phase having crystal orientation aligned in a cold-working direction, and wherein the crystal orientation density measured by an electron back scattering pattern method is 2.0 or more in the cold-working direction.
The copper-based alloy is formed by repeating a cycle of annealing and cold-working a plurality of times. The crystal orientation of the xcex2-single phase is preferably  less than 110 greater than  or  less than 100 greater than  orientation. The copper-based alloy is preferably produced by a method comprising a plurality of solution treatments to improve the crystal orientation density in the cold-working direction.
The copper-based alloy preferably has a composition comprising 3 to 10 weight % of Al, 5 to 20 weight % of Mn, the balance being substantially Cu and inevitable impurities. The composition may further comprise at least one metal selected from the group consisting of Ni, Co, Fe, Ti, V, Cr, Si, Nb, Mo, W, Sn, Sb, Mg, P, Be, Zr, Zn, B, C, Ag and misch metals in a total amount of 0.001 to 10 weight % based on the copper-based alloy of 100 weight %.
A method for producing the copper-based alloy according to the present invention comprises the steps of: repeating a cycle of annealing and cold-working to an alloy a plurality of times, a maximum cold-working ratio being 30% or more in the cold-working; and subjecting the cold-worked alloy to a solution treatment at least once, a quenching and an aging treatment.
After the solution treatment, the alloy is preferably cooled down to a temperature range, at which microstructure thereof is transformed into xcex2+xcex1 dual phase, and subjected to another solution treatment. In particular, it is preferable that a cycle of the solution treatment and cooling is carried out twice or more, and that the final cooling is rapid cooling. It is preferred that the annealing is carried out to transform a microstructure of the alloy into such that comprising xcex1-phase of 20 volume % or more before each cold-working. The maximum cold-working ratio is generally 30% or more, particularly preferably 50% or more in the cold-working.
A wire according to the present invention is made of the copper-based alloy having an average grain diameter equal to or more than the radius of the wire. The average grain diameter is preferably twice or more to the wire diameter. Further, a region of the wire, in which the copper-based alloy has a grain diameter equal to or more than the radius of the wire, preferably has a length of 30% or more based on the entire length of the wire. Specifically, in the case of the wire having the radius of 0.25 mm, it is preferably that 30% or more of the entire crystal grains has a diameter of 0.3 mm or more. The wire of the present invention may be used as guide wires for catheters, twisted wires, etc. The wire can be produced by a method comprising the steps of: repeating a cycle of annealing and cold-working to an alloy a plurality of times to form the alloy in a wire shape having a predetermined diameter; and subjecting the cold-worked alloy to a solution treatment at least once, a quenching and an aging treatment.
A plate or foil according to the present invention is made of the copper-based alloy having an average grain diameter equal to or more than a thickness of the plate or foil. The average grain diameter is preferably twice or more to the thickness of the plate or foil. Further, a region of the plate or foil, in which the copper-based alloy has a grain diameter equal to or more than the thickness of the plate or foil, preferably has an area of 30% or more based on the entire area of the plate or foil. Specifically, in the case of the plate or foil having the thickness of 0.5 mm, it is preferably that 50% or more of the entire crystal grains has a diameter of 0.5 mm or more. The plate or foil of the present invention may be used for connector members, clips for writing implements, etc. The plate and foil can be produced by a method comprising the steps of: repeating a cycle of annealing and cold-working to an alloy a plurality of times to form the alloy in a plate or foil shape having a predetermined thickness; and subjecting the cold-worked alloy to a solution treatment at least once, a quenching and an aging treatment.
A pipe according to the present invention is made of the copper-based alloy having an average grain diameter equal to or more than a thickness of the pipe. A region of the pipe, in which the copper-based alloy has a grain diameter equal to or more than the thickness of the pipe, preferably has an area of 30% or more based on the entire area of the pipe. The pipe can be produced by a method comprising the steps of: forming an alloy in a pipe shape by hot extrusion, etc.; repeating a cycle of annealing and cold-working to the formed alloy a plurality of times; and subjecting the cold-worked alloy to a solution treatment at least once, a quenching and an aging treatment.